black missing on last 3rd of large format photo

[redeyereeves]

Hello, I'm at the 11th hour, trying to print a black & white 2m x 1m photo on matte canvas for an exhibition... I'm using an imac with CS3 and an HP Z3100. The first print was perfect until the last 3rd of the print - which came out with no deep blacks. I did a diagnostic test print, cleaned the jets, and also replaced the light grey cartridge as it was flashing empty. After another test print all seemed fine, so I tried another big print but exactly the same thing happened at exactly the same place. It's not a big file... only 16 MB, and I have printed another photo to this size before that was 35 MB and that was fine.
Does anyone have any ideas as to what is causing the black cartridge to seemingly switch off at the same point on this particular photo? Any help would be much appreciated.